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The first radio call from PhD candidate Christopher Pugh on a Twin Otter plane alerted the research team from the Institute for Quantum Computing (IQC) on the ground in Smiths Falls, Ontario that the receiver on the aircraft was receiving photons from the source on the ground. It was the first, crucial sign that the demonstration was going to be a success.

The team led by Thomas Jennewein, a faculty member with IQC and the Department of Physics and Astronomy at the University of Waterloo, was testing the viability of the Quantum Key Distribution (QKD) system that they had custom-built for a satellite mission. Working together with National Research Council Canada (NRC), the team set up their receiver on the NRC Twin Otter Airborne Research Aircraft that would fly out of Ottawa and prepared a ground station at Smiths Falls-Montague airport.

On the ground, the custom-built source that we call Alice randomly prepared photons in four different polarization states - horizontal, vertical, +45 degrees and –45 degrees. Alice then sent them to the custom-built receiver (Bob) aboard the NRC aircraft. Bob detected the photons that weren’t lost in the beam spread, analyzed the polarizations and recorded the time. Alice and Bob then compared their data to determine which photons made it and their polarization states to construct the secret key. Analysis showed the channel was safe and they could encrypt their message securely.

An eavesdropper, who we call Eve, cannot extract information because of quantum mechanics. She would need to intercept one of the photons, replicate its exact properties including the polarization state and send it along. Not only would it require a satellite in between Alice and Bob, but the laws of quantum mechanics state that once you observe a system, you perturb it, so Eve would be immediately detected. Another advantage of generating the key between Earth and a satellite is that there is no absorption loss outside the atmosphere compared to the loss on the ground through optical fibre limiting distances to a few hundred kilometres.

“The system we tested is entirely based on hardware that would go on a satellite for long-distance communication,” said Jennewein. “Not only did we demonstrate that the prototype itself works, but the team exchanged more than a million bits of the secure key during seven flyovers.”